Food products of various kinds and types are frequently frozen in cryogenic freezing tunnels; such a freezing tunnel comprises an elongated housing with a food product entrance at one end and an exit at the other end and a conveyor extending throughout the length of the tunnel. A spray header discharges a liquified cryogen gas, usually liquified nitrogen, onto the food product at a cryogen input position near the food product exit. The cryogen gas is exhausted from the entrance end of the tunnel through an exhaust stack equipped with an exhaust blower. Throughout the length of the tunnel, circulating fans deflect the cryogen gas into contact with the food product. Practical and advantageous mechanical constructions for cryogenic food product freezing tunnels of this kind are disclosed in Kent U.S. Pat. No. 3,757,533 issued Sept. 11, 1973 and Morgan et al U.S. Pat. No. 4,171,625 issued Oct. 23, 1979. A highly advantageous tunnel control system is disclosed in Sandberg U.S. Pat. No. 4,142,376 issued Mar. 6, 1979. Other prior art of interest in connection with the present invention, concerned primarily with control of the flow of cryogen gas within the tunnel, includes Berreth et al U.S. Pat. No. 3,403,527 issued Oct. 1, 1968 and Klee et al U.S. Pat. No. 3,892,104 issued July 1, 1975.
A basic problem in the operation of any such continuous-flow cryogenic freezing tunnel derives from leakage of the cryogen gas into the workspace in which the tunnel is located through either the product exit end or the product entrance end, since both are partially open during operation. In part, this is a problem of control of the flow of cryogen into the tunnel; whenever the input of liquified nitrogen or other cryogen is excessive, a cold gas discharge from either or both ends of the tunnel may occur. The leakage problem is also partly one of proportionate flow within the tunnel. In particular, too much cryogen gas flow toward the product exit end of the tunnel can create leakage even if the inflow of liquified cryogen is not excessive. Gas leakage presents a substantial problem in maintenance of an appropriate ambient temperature in the workspace around the tunnel and may present a health hazard to workers near the tunnel.
Another continuing problem of substantial difficulty, relative to continuous-feed cryogenic freezing tunnels, pertains to efficiency in use of the cryogen. If too little cryogen is provided, the freezing of the food product is inadequate, with a consequent loss of quality and possible spoilage. If too much cryogen is supplied, on the other hand, the tunnel operation becomes economically wasteful, even if the tunnel exhaust successfully prevents escape of the cryogen gas into the workspace adjacent the tunnel, Again, the efficiency problem is not merely one of control of the quantity of liquid cryogen entering the tunnel; the flow of cryogen gas within the tunnel and the rate of removal of gas from the tunnel through the exhaust stack are both important factors affecting the efficiency of tunnel operation.
Some partial solutions to these difficulties are presented in the prior art noted above. Thus, the Berreth et al and Klee et al patents provide variable speed blowers and/or variable damper arrangements within the tunnel itself to control the proportionate flow of gas from the cryogen input location toward the exit and entrance ends of the tunnel, based primarily upon sensing of the rate of input of the liquified cryogen. The Morgan et al patent incorporates adjustable dampers in the entrance end of the tunnel, as a part of the tunnel exhaust system, controlling the exhaust of cryogen gas from the tunnel. That damper arrangement, however, causes variable withdrawal of air from the workspace around the tunnel, and consequently may impose substantial burdens on heating or cooling systems serving that workspace, depending upon the geographic location and the time of year.